The iron uptake of Japanese has been maintained at about 100% sufficiency without change since 1975, and iron as a nutrient in meals must be taken up with care. In the world, iron is considered as a nutrient which is often liable to be deficient, particularly the supply of iron in enriched foods and medicines for persons with anemic tendency, and pregnant and nursing females. However, addition of an iron salt such as iron sulfate or citrate as an iron reinforcing agent in foods and drinks has drawbacks since it cause problems of characteristic astringent taste when added in foods and drinks and injury to gastrointestinal mucosa, thus the amount of addition is limited. Furthermore, an organic iron compound, heme iron, causes problems in taste such as metallic or fishy taste and its addition to foods is highly limited.
Addition of milk casein, amino acid or casein phosphopeptide has been tried for the improved absorption of iron [Japanese Laid-open Patent Application No. 162843 (1984)]. However, these methods could not diminish characteristic astringent taste of iron without reducing the amount of its addition.
The inventors of the present invention developed a method to reduce the characteristic astringent taste of iron by binding iron with casein [Japanese Laid-open Patent Application No. 83400 (1990)]. However, the iron casein prepared by binding iron with casein is devoid of thermostability and shows characteristic astringent taste of iron by thermal pasteurization at 90.degree. C. for 10 minutes, 120.degree. C. for 2-3 seconds or retort pasteurization. This was considered to be caused by release of iron from casein due to weak binding of iron and casein, and formation of iron hydroxides or the like.
Then, the inventors of the present invention further investigated and found that the binding of iron and casein can be reinforced by using carbonic acid and/or hydrogencarbonic acid and succeeded to get carbonic acid- and/or hydrogencarbonic acid-iron-casein complexes [Japanese Laid-open Patent Application No. 259572 (1995)]. The carbonic acid- and/or hydrogencarbonic acid-iron-casein complexes are thermoresistant without showing iron characteristic astringent taste even by heat sterilization and are useful as raw materials of foods and drinks, medicines, feeds and so forth for prevention and treatment of anemia and reinforcement of iron content. However, the carbonic acid- and/or hydrogencarbonic acid-iron-casein complexes had drawbacks of tendency to aggregate by dissolution in water, difficult to treat in the production procedure and liable to lose solid mass. Furthermore, their sandy taste remains to be solved.
The inventors of the present invention actively investigated methods to solve the above mentioned drawbacks of carbonic acid- and/or hydrogencarbonic acid-iron-casein complexes, and found that carbonic acid- and/or hydrogencarbonic acid-iron-partial hydrolyzates of casein complexes prepared by hydrolysis of carbonic acid- and/or hydrogencarbonic acid-iron-casein complexes with a protease, or by hydrolysis of casein with a protease to give partial hydrolyzates of casein followed by preparation of carbonic acid- and/or hydrogencarbonic acid-iron-partial hydrolyzates of casein complexes provides thermoresistant, stable to heat sterilization products without iron characteristic astringent taste or aggregation by dissolution in water and accomplished the present invention. Therefore, one object of the present invention is to provide carbonic acid- and/or hydrogencarbonic acid-iron-partial hydrolyzates of casein complexes having characteristic features of thermoresistant, stability to heat sterilization without iron characteristic astringent taste or aggregation by dissolution in water. In addition, the present invention provides processes for the preparation of carbonic acid- and/or hydrogencarbonic acid-iron-partial hydrolyzates of casein complexes.